In a wireless communication system, in order to provide various broadband information services, it is desirable at all times that transfer speed is improved. It is possible to realize an improvement in the transfer speed by broadening a communication bandwidth, but because there is a limit in an available frequency band, an improvement in frequency efficiency is indispensable. As a technology for greatly improving frequency efficiency, a multiple input multiple output (MIMO) technology that performs wireless transfer using multiple transmit and receive antennas is attracting attention, and is practically used in a cellular system, a wireless LAN system, or the like. An amount of improvement in the frequency efficiency due to the MIMO technology is proportional to the number of the transmit and receive antennas. However, there is a limit in the number of the receive antennas that may be in a terminal device. Then, multi-user MIMO (MU-MIMO) in which multiple terminal devices that make connections at the same time are regarded as a virtual large-scale antenna array, and transmission signals from the base station device to each terminal device are space-multiplexed is effective in improving the frequency efficiency.
In MU-MIMO, because the transmission signals that are destined for the terminal devices, respectively, are received in the terminal device, causing inter-user-interference (hereinafter referred to as IUI), IUI needs to be suppressed. For example, in Long Term Evolution that is employed as one of the 3.9-th mobile wireless communication systems, linear precoding is employed in which multiplication by a linear filter that is calculated based on channel state information that is notified by each terminal device is performed in advance in the base station device and thus the IUI is suppressed.
Furthermore, as a method of realizing MU-MIMO with which much greater improvement in frequency efficiency can be expected, a MU-MIMO technology that uses non-linear precoding in which non-linear processing is performed on the side of the base station device has attracted attention. In a case where a modulo operation is possible in the terminal device, it is possible to add a perturbation vector of which an element is a complex number (a perturbation term) that results from multiplying an arbitrary Gaussian integer by a fixed real number, to the transmission signal.
Then, if the perturbation vector is appropriately set according to a channel state between the base station device and each of multiple user devices, it is possible to reduce the needed transmission power more greatly than in the linear pre-coding. As the non-linear pre-coding, vector perturbation (VP) disclosed in NPL 1 or Tomlinson Harashima precoding (THP) disclosed in NPL 2, which are schemes with which optimal transmission performance can be realized, is well known.
Incidentally, because the pre-coding is performed according to the channel state between the base station and the terminal device, the precision of the pre-coding depends greatly on the precision of channel state information (CSI) which the base station can be aware of. In the wireless communication system that depends on frequency division duplex that uses different carrier frequencies in downlink transfer and uplink transfer, the CSI estimated by the terminal device is fed back to the base station device, and thus the base station device can be aware of the CSI. However, there is a likelihood that an error will occur between the CSI that the base station device can be aware of and actual CSI. This problem is briefly described referring to FIG. 11.
FIG. 11 is a sequence chart illustrating a situation of communication between the base station device that performs the pre-coding and a terminal device. First, the base station device transmits a reference signal for estimating the CSI to the terminal device (Step S1). Furthermore, the base station device generates transmission data and a demodulation reference signal (Step S2). Because the reference signal is already known to the base station device and the terminal device, the CSI can be estimated based on the received reference signal (Step S3).
However, practically, because noise is necessarily applied to the received signal, an error occurs between the estimated CSI and real CSI. The terminal device converts the estimated CSI into information that is available for notification to the base station device, and notifies the base station device of the resulting information (Step S4). As the information that is available for the notification, information that results from quantizing the estimated information directly into digital information, a number indicating a code listed in a code book that is shared between the base station device and the terminal device, or the like, is given. The base station device restores the CSI with the notified information, but an error occurs between the restored CSI and the real CSI, too. The error between the real CSI and the CSI that the base station device is finally made to be aware of is hereinafter referred to as a quantization error. Thereafter, in the base station device, the pre-coding is performed based on the restored CSI (Step S5), and the data transmission to the terminal device is performed (Step S6).
When receiving data from the base station device, the terminal device performs channel estimation for demodulation (Step S7), performs channel equalization (spatial signal detection processing) (Step S8), and demodulates the transmit data (Step S9). At this point, because the terminal device estimates the CSI, a fixed processing delay time (also referred to as round trip delay) occurs until the base station device performs the pre-coding processing and transmits a signal. Normally, because time selectivity is present in a channel, an error occurs between the CSI that is propagated by a signal on which the pre-coding is performed, and the CSI estimated by the terminal device. The CSI error that occurs depending on the time selectivity in the channel is hereinafter referred to as a time change error and the quantization error and the time change error are hereinafter collectively referred to as a feedback error. Because the feedback error is present in the CSI that the base station device can be aware of, it is extremely difficult for the base station device to acquire high-precision CSI.
On the other hand, in a wireless communication system that depends on time division duplex that uses the same carrier frequencies in the downlink transfer and the uplink transfer, the feedback error occurs as well, as is the case with the frequency division duplex. This problem is briefly described referring to FIG. 12. FIG. 12 is a sequence chart illustrating a situation of the communication between the base station device that performs the pre-coding and the terminal device. In the time division duplex, the transmission is performed in a state where the uplink transfer and the downlink transfer are divided in terms of time. First, the uplink transfer from the terminal device to the base station device is performed (Step T1). At this time, a reference signal for signal demodulation is included in a signal for the uplink transfer, and the base station device acquires the CSI from the reference signal and performs signal demodulation (Step T2).
Subsequently, it is considered that the base station device performs the pre-coding on a signal for the downlink transfer. At this time, because duality is present between a channel for the uplink, and a channel for the downlink in the time division duplex, the base station device can perform the pre-coding based on the CSI that was acquired some time ago to demodulate the signal for the uplink transfer (Step T3). Then, data is transmitted to the terminal device (Step T4). On the other hand, in the terminal device, the channel estimation and the downlink signal demodulation are performed (Step T5).
However, generally, because multiple signals for the uplink transfer and multiple signals for the downlink transfer are alternately transmitted, the time change error is present between the CSI that is propagated by a signal that is transmitted, which is in the latter half of the multiple signals for the downlink transfer, and the CSI that is used in the pre-coding. Furthermore, because the duality is present in the channel itself and on the other hand, the duality is not present in analog circuits of the base station device and the terminal device, the CSI for the uplink and the CSI for the downlink are not necessarily the same. CSI errors that occur in this manner are hereinafter collectively referred to as the feedback error.
As described above, in order to improve the channel performance of the pre-coding transfer in an environment where an influence of the CSI feedback error is great, NPL 3 discusses a method in which the terminal device estimates channel state information anew at a point in time at which a reception signal on which the pre-decoding is performed is received in the terminal device, and based on the channel state information, performs approximate channel equalization processing anew, thereby lessening degradation in the channel performance due to the feedback error. However, in the method in NPL 3, a case where only one data stream is sent to each terminal device is assumed and only the linear pre-coding is considered for the pre-coding.